1. Field of the Invention
The present invention relates to an offset printing method utilizing a fine-line printing technology for forming a fine conductive wiring pattern on a circuit board, etc. by printing, and more particularly, utilizing the transfer method and an offset printing machine to be employed for said offset printing method.
2. Description of the Prior Art
Accompanying the miniaturization of electronic devices, multi-layering and high density packaging of circuit boards to be employed therefor has been required. In order to satisfy this requirement, it is indispensable to miniaturize the wiring of the circuit board. In recent years, by the development of the COB (Chip on Board) technology, the wiring on the circuit board has been further miniaturized and a fine wiring of 50 .mu.m width lines at 100 .mu.m pitch has been required.
Conventionally, in the fabrication of a hybrid IC employing a ceramic substrate and multilayered ceramic substrate, the wiring of Cu or Ag/Pd of a comparatively low resistivity by which the resistance of conductive wiring can be made low has been formed by employing the screen printing method which is simple and capable of a thick film printing. The screen printing method is already known and the most widely used method excellent in the productivity.
However, in the conventional screen printing method, a fine wiring printing of 75 .mu.m width line and 150 .mu.m pitch is the limit of the method, and further when a printing failure such as overlap between printed lines or cut-off of printed line takes place, the substrate has to be discarded. In printing on the substrate, overlap between printed lines takes place due to a large output volume of ink or ink flow. The printed line cut-off is more likely to take place in a more fine line. This is caused because ink can not get access to the non-opening portion of the screen mesh where ink is not discharged. Although it is reported in the research level (ISHM 1990 Proceedings pp. 445-452) that fine lines of 30 to 50 .mu.m width were printed by the screen printing method, the screen printing method is likely to cause overlap between printed lines, and many problems are left to be solved before it is introduced into the mass production. Besides the printing method, the inspection method, and the discard of defective printing substrate are also big problems.
Furthermore, the screen printing method has also a problem that when the substrate has a surface irregularity, printing can not be provided over the entire surface of the substrate. In order to solve this problem, the transfer printing method is effective. As the technology relating to this method, there are available, for example, a transfer method employing a transfer sheet having a release layer and an adhesive layer (U.S. Pat. No. 5,069,954) and a solvent-free thermoplastic ink (U.S. Pat. No. 4,472,537 and U.S. Pat. No. 4,426,227) to be employed for the transfer with silicone rubber. With mere employment of these methods and ink, it is difficult to form a thick film printing line of 30 to 50 .mu.m width, and thus a new technology is required.
Further, in Japanese patent publication Hei 1-17278, there is proposed an apparatus for printing on curved surfaces wherein a thin strip-like rubber is used as a transferring medium and a print mechanism for printing thick paste on said strip-like rubber, a transferring device for pressing said strip-like rubber onto a substance to be printed and a positioning device for holding said substance having curved surfaces.
However, this invention is intended to print thick paste on curved surfaces, for example, edges of a substrate and not intended to minimize the width of each line to be printed.